contrib/ciabot: git-describe commit instead of HEAD

contrib/ciabot: git-describe commit instead of HEAD

For each commit a shorter version of the name will be generated. This is
either the truncated hash or the output of git-describe. The
call to git-describe was only made with an empty shell variable instead
of an actual commit hash. Thus it only described the current HEAD and
not each commit we want to submit to cia.vc.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>